1. Field of the Invention
This invention relates generally to separation, storage, and analysis of ions according to ion mobilities of charged particles and charged particles derived from atoms, molecules, particles, sub-atomic particles and ions. More specifically, the present invention is an ion mobility analyzer that is employed to detect a wide range of chemicals, wherein the analyzer differentiates chemical compounds based upon their ion mobilities.
2. Description of Related Art
To understand the advantages of the present invention, it is useful to examine the state of the art of mass spectrometry. Chemical analysis of charged particles and charged particles derived from ions, molecules, particles, sub-atomic particles and atoms (hereinafter to be collectively referred to as ions) can be done by separating their ionic forms according to their mass-to-charge ratios. There are various kinds of mass spectrometers. Each mass spectrometer has been found to have its own special characteristics and applications, as well as limitations. In the case of time-of-flight (TOF) mass spectrometry, the TOF mass analyzer measures the mass-to-charge (m/z) dependent time that it takes for ions of different mass-to-charge ratios to move through a flight tube from an ion source to a detector. The analysis is based on measurements of the flight time required for the ion to move along a tube of a defined length in an environment that is free of electric fields.
Time-of-flight mass spectrometry performs its analysis based on the characteristics of charge and mass of ions. In contrast, a related technique known as ion mobility mass spectrometry (IMS) is dependent upon the charge, size and shape (the cross-sectional area) of molecules to perform its analysis of ions.
IMS is a gas phase electrophoretic separation technique in which ions are separated based on their ionic mobilities as the ions drift through a buffer gas under the influence of an electric field. The analysis is based on measuring the drift time that it takes for the ion to move along a drift tube of a defined length in an applied electric field.
There are different types of IMS instruments that need to be understood in order to understand the principles of the present invention. In conventional IMS, an electric field produces a linear relationship between the drift velocity and electric field. Accordingly, reduced mobility is generally independent of the electric field. A sample is introduced into an ionization region containing an ion source. Ionized samples are then accelerated into a drift region in a drift tube, often with a buffer gas introduced from the opposite direction. Ions are separated as they drift through this buffer gas. Separation of the ions is based upon size, shape, and charge of the ions. Ions that drift through the buffer gas are registered at the detector. Conventional IMS systems generally use a low electric field and are characterized by having a low duty cycle.
In differential mobility analysis (DMA), ions are separated according to their mobilities by the application of an electric field and a flow field that are orthogonal to each other. The ions of different mobilities are dispersed in space so that only ions of a selected mobility pass through a detector slit. DMA is often used in aerosol experiments to analyze particles of a given size.
The last type of mobility analyzer is known as high-field asymmetric waveform ion mobility spectrometry, or FAIMS. In FAIMS, two concentric tubes or plates are generally used. A high electric field is applied for a short time, and then a low electric field is applied for a longer duration, with the average applied electric field being balanced. The non-linearity of the FAIMS system is generally attributed to the different cross-sectional areas of the ions that are drifting through the tube or between the plates. Accordingly, the method takes advantage of the different mobilities of ions in a high electric field as compared to a low electric field.
Another way of describing FAIMS is to say that the separation of ions is based on the nonlinear dependence of the mobility constant with respect to the electric field intensity. The change in mobility at high electric field values appears to reflect the size of the ion, its interaction with the buffer gas, and the structural rigidity of the ion. Thus, the ratio of high electric field mobility to low electric field mobility is used in the characterization of ions in FAIMS.
IMS as described by the three techniques above is a relatively fast method of ion analysis, is highly sensitive, moderately selective, and has a low limit of detection. However, IMS has generally received little attention because of its relatively poor resolution, limited linear dynamic range, and the previously mentioned low to moderate selectivity.
Accordingly, what is needed is a new form of ion mobility spectrometry that overcomes the disadvantages of the existing IMS methods. Specifically, it would be an advantage over the state of the art of IMS to be able to provide increased sensitivity, increased resolution, more accurate mobilities and specific detection.